Nowadays, more and more detection devices implement Time-Of-Flight (TOF) technologies for obtaining depth information. A basic Time-Of-Flight (TOF) camera system 3 is illustrated in FIG. 1. TOF camera systems capture 3D images of a scene 15 by analysing the time of flight of light from a light source 18 to an object. TOF camera system 3 includes a camera with a dedicated illumination unit 18 and data processing means 4.
The operational principle of a TOF camera system is to actively illuminate the scene 15 with a modulated light 16 at a predetermined wavelength using the dedicated illumination unit, for instance with some light pulses of at least one predetermined frequency. The modulated light is reflected back from objects within the scene. A lens 2 collects the reflected light 17 and forms an image of the objects onto an imaging sensor 1 of the camera. Depending on the distance of objects from the camera, a delay is experienced between the emission of the modulated light, e.g. the so called light pulses, and the reception at the camera of those light pulses. Distance between reflecting objects and the camera may be determined as function of the time delay observed and the speed of light constant value. In another more complex and reliable embodiment, a plurality of phase differences in between the emitted reference light pulses and the captured light pulses may be determined by correlation measurement and used for estimating depth information.
The determination of the phase differences can be carried out notably by Current-Assisted Photonic Demodulators (CAPDs). The principle of CAPDs is explained in EP1513202 and illustrated by FIGS. 2A-C. It is based on demodulation nodes, the so-called “taps”. The CAPD represented on FIGS. 2A-C comprises two taps. Each tap consists of a control region 61, 62 and a detection region 63, 64. By controlling a potential applied between the control regions 61 and 62, it is possible to control the detectivity of the associated tap. When a photon is incident on the photosentitive area of a pixel, an electron-hole e−/h+ pair may be generated at a certain position. The electron-hole pair will be separated by an electrical field that is present and that is associated with the flowing majority current. This electrical field will cause the photogenerated minority carriers 66, 69 to drift in the opposite direction to the flowing majority current, i.e. towards the detection regions 63, 64, respectively.
When a pixel comprises several taps and when a positive potential is applied to a tap with respect to the other taps, this tap is activated and will be receiving the majority of the photogenerated minority carriers in the pixel, as illustrated by FIGS. 2B and C. By applying appropriate driving signals to the control regions, correlation measurements can be performed and the depth perception can be obtained.
Prior art CAPDs suffer from several drawbacks to be overcome. A first challenge in CAPDs is to reduce the size of the pixels while avoiding crosstalk phenomenon i.e. parasitic charge exchange between neighbouring pixels. This crosstalk can indeed cause a loss in image quality.
Another challenge in CAPDs is to create a field between the control regions as high as possible in order to achieve a high detectivity and a high demodulation contrast. This requirement involves high power consumption; this is one of the main disadvantages of CAPDs. The power consumption P in a CAPD follows the following equation, R and ΔV being the resistance and the potential difference between the control regions, respectively:
  P  =                    R        ⁡                  (                                    Δ              ⁢                                                          ⁢              V                        R                    )                    2        =                  Δ        ⁢                                  ⁢                  V          2                    R      
The power consumption P can be reduced for instance by increasing the distance between the control regions in order to increase the resistance between them. Nevertheless, this solution suffers from the drawback of negatively affecting the size of the device.
Another challenge in CAPD devices is to improve the data binning methods for obtaining more reliable data. Indeed, in regular binning, each pixel is typically read and the information is then added. This requires more time for the higher read-out count and adds the read-out noise several times.
A solution remains to be proposed in order to decrease the power consumption of CAPDs while reducing the size of the pixels, avoiding cross talk phenomenon between the pixels and allowing an improved data binning.